think some have misunderstood the question.
You want to know how high the cloudbase will be when you only know the temp and dewpoint?
I don't know the answer, but I know a man who may do.
The 'Wendywindblows' automatic weather stations (as used by hang gliders) works out cloudbase height this way. Find the man, and there is your answer. I'll dig about - if that's the question.
Errrm, well that is what I (me me me me) has answered??
2.5/1000'. So if yer temp is 8/4 then the cloud base will be at 1600' ((8-4)*1000/2.5 you see).